Sample and hold method to achieve square-wave PWM current source for light emitting diode arrays

ABSTRACT

An apparatus for controlling overshoots in the switching of LED arrays in a system having a switching voltage converter providing the biasing voltage for the LED array. By synchronizing a switching converter to an LED turn-on signal, loading on the voltage converter can be controlled such that output conduction of the converter only occurs when LEDs that are to be displayed are switched on to provide loading to the output of the converter. A Sample and hold method is employed to effectively store the current information in a previous “on” interval and use it for the current control in a following interval with inhibited current overshoot.

FIELD OF THE INVENTION

[0001] This invention relates to the field of LED drive circuits, andmore particularly to a method for driving an LED array using a sampleand hold method to achieve square wave PWM current waveform required byLED loads.

BACKGROUND OF THE INVENTION

[0002] Conventional LED array driver circuits typically use aninexpensive linear voltage regulator to provide a V_(cc) biasing rail asan energy source for LEDs to be displayed. When an LED is selected forlighting, a semiconductor switching device, such as a transistor, isactivated to provide a current path through the selected LED(s). At theend of the display period, the device is then turned off. A significantdrawback of such a voltage driving arrangement is that voltageovershoots occur at the time of turn off of the switching device due tocontinued conduction of the source regulator at the instant of turn off.This event presents a condition wherein a large filtering capacitor isreceiving charge from the linear regulator at the instant of removal ofthe LED loading. Then, at the next turn-on cycle of the LED, a highervoltage is present on the voltage biasing rail, causing large leadingcurrent spikes at that voltage transition. These overshoots can beinjected into neighboring circuitry with degrading or destructive effectboth to the circuits and the LED.

[0003] To eliminate such overshoot problems, circuits are configured tocreate a constant current source from a linear voltage regulator. FIG. 1shows such a conventional LED driver circuit 10, having a firstamplifier 12 for amplifying an I_(LED) signal present at node 14 and asecond amplifier 16 for controlling the operation of the LED. Inresponse to a logical switching signal 18, second amplifier 16 presentsa regulated voltage signal 20 to transistor 22 to begin currentconduction through LEDs 24. The resulting LED current, I_(LED), issensed via the sensing resistor 26 and is amplified by first amplifier12 to provide a current feedback signal to second amplifier 16. TheLaplace transfer function of the second amplifier is governed by theequation $\begin{matrix}{\frac{R_{A}}{R_{B}}\frac{1}{1 + {s\quad R_{A}C_{1}}}} & \lbrack 1\rbrack\end{matrix}$

[0004] A linear time delay that is provided by the R_(A)C₁ impedancecombination implements a low pass filter that reduces overshoot inI_(LED), thereby allowing current signal I_(LED) to follow referencesignal 18. A disadvantage of such linear low pass filtering is the largeovershoot in LED current during the turn-on of LEDs 24. For applicationsrequiring LEDs 24 to be displayed using a pulse width modulated (PWM)mode at a high frequency, such as 400 Hz, and an exemplary duty cycle of40%, substantial ripple current results in I_(LED) that can causedegradation in the optical and electrical performance of the LEDs 24.This degradation can include loss of intensity control and accuracy, inaddition to creation of noise signals that can interfere with thesampling scheme.

[0005] Thus, a need exists for an apparatus that can inhibit theovershoots while allowing high speed selectivity of the devices in anLED array.

SUMMARY

[0006] According to a preferred embodiment of the present invention, acircuit is provided for synchronizing the current feedback and controlsignals of an LED driving circuit with a second signal for driving abiasing power converter in order to eliminate voltage and currentovershoots associated with LED loading discontinuities. By suchsynchronization, a switching device in the power converter which isconnected to an energy source is inhibited during times when the LEDload driving device is inhibited to prevent undesired charging ofintermediate buffering capacitances. Through the use of gate clampingdevices, a single logic signal, with inversion where appropriate, cancause all switching devices to turn-on and turn-off simultaneously. Thisinvention is applicable to both flyback converters and push-pullconverters.

[0007] Further, a sample and hold circuit preferably provides for anoperating biasing level that will insure that a subsequent turn-on willhaving the same operating conditions that were present at a previousturn-on. The response time of such a sample and hold circuit willpreferably be longer than the time period of the operating frequency ofthe converter.

[0008] The present invention also provides a method for synchronizingthe switching LED driver to the switching device in the voltageconverter such that both the activation time periods and theinactivation time periods coincide, and that no energy transfer isenabled if an energy load is not present.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009]FIG. 1 shows a conventional LED driver circuit.

[0010]FIG. 2 shows an LED driver configuration according to a preferredembodiment of the present invention.

[0011]FIG. 3 shows an exemplary detailed schematic of an LED switchingcontrol circuit according to another embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

[0012] According to a preferred embodiment of the present invention, anon-linear sample and hold circuit allows for an accurate measurementand correction of an LED current signal, such that a driving signal 18will be accurately followed without undue voltage and current overshootand performance degradation.

[0013]FIG. 2 shows an exemplary LED driver configuration 28 according toa preferred embodiment of the present invention. An amplified sensesignal 30 is applied to an exemplary wave-shaping impedance combinationof diode 32, capacitor 34, resistor 36, and the drain of MOSFETswitching device 38, the input gate of which is controlled by an inputvoltage signal 40. A second MOSFET switching device 42 is alsocontrolled by input signal 40 via logic inverter 44. This secondswitching device 42 provides a current sinking circuit through resistor46 connected to the output of amplifier 48 and the gate of transistor22. Amplifier 48 is biased similarly to amplifier 16 shown in FIG. 1,except that the positive input terminal is connected to a fixedreference voltage 50.

[0014] Due to inverter 44, switching devices 38 and 42 will be inopposite states in response to logic signal 40. When signal 40 is at ahigh voltage, switching device 38 turns on and switching device 42 turnsoff, thus allowing amplified signal 30 to pass to amplifier 48 andthence to output transistor 22, thereby turning on LED's 24. Thefeedback signal through the just describe loop regulates the currentthrough LEDs 24 at an essentially constant magnitude for the duration ofthe applied pulse.

[0015] When signal 40 transitions low, switching device 38 turns off andswitching device 42 turns on to pull the gate voltage of transistor 22and its associated parasitic gate capacitance 52 to a level that isbelow its threshold, thereby turning off transistor 22 and LED's 24. Thebiasing resistors associated with the two switching devices 38 and 42can be selected such that transistor 22 is only slightly below itsthreshold voltage. This allows turn on of transistor 22 without the needfor gate overdrive levels that can produce the current overshoots inLEDs 24, since capacitors 34 and 52 are never completely discharged norfully charged during the cyclic operation.

[0016]FIG. 3 shows an exemplary detailed schematic of an LED switchingcontrol circuit 54 according to another embodiment of the presentinvention. An exemplary input voltage of 15 volts DC at rail 56, isconverted by switching device 58 and flyback transformer 60 to provide aDC supply voltage to node 62, capacitor 64, and LEDs 66. A standard UPCdevice 68 controls the pulse-width-modulated (PWM) signal to switchingdevice 58, such that a constant regulated voltage that is independent ofinput voltage 56 is applied to capacitor 64. The input voltage andconverter configuration shown in FIG. 3 is exemplary only, and not meantto be restricting. Many different configurations and input voltages canbe used with the same effect, for example, an AC rectified line voltagebeing converted using a push-pull converter or inverter arrangement.

[0017] Completing the current path is transistor 70, which connects LEDs66 to sensing resistor 72 and the ground return. A voltage signal 74that is proportional to the current through LEDs 66 is then sensed bylinear amplifier module 76, which amplifies the signal provides theamplified signal to a peak detector circuit comprising diode 78,capacitor 80 and resistor 82. By selecting the RC time constant of thepeak detector circuit to be larger than the frequency of the PWM controlcircuitry, a voltage signal results that is proportional to the peakcurrent in LEDs 66.

[0018] During an on state of transistor 70, the peak current in LEDs 66can be controlled by this voltage level in order to maintain a desiredconduction current. During off state of transistor 70, the sensedcurrent drops to zero almost instantaneously and can cause the nowunloaded voltage on capacitor 64 to rise and overshoot due to the slowresponse time of the control loop. Accordingly, UPC device 68 must beinhibited in synchronization with transistor 70 turn-off, which isimplemented via transistors 84 and 86 and input PWM control signal 88.

[0019] Digital PWM control signal 88 thus passes through amplifierdevice 76 and controls the above transistors, such that when transistor70 is on, transistor 84 is on and transistor 86 is off and UPC device 68and the peak detector circuit operate normally. In the exemplary circuitshown in FIG. 3, this state corresponds to a high logic state at controlsignal 88. When a logic low is present at PWM signal 88, transistor 70turns off and transistor 86 turns on, thereby inhibiting the operationof UPC device 68 and the peak detector circuit. During the on time oftransistor 86, the peak detector circuit stores the previously retainedvoltage value, and the voltage on capacitor 64 remains constant, sincethere is no discharge path for the duration of the PWM signal 88 beingat a logical low level. (i.e. transistor 70 being in an “off” state).When the next logical high level on PWM signal 88 occurs, transistor 70turns on at the same level as before the turn-off, with the result thatno overshoot occurs.

[0020] Numerous modifications to and alternative embodiments of thepresent invention will be apparent to those skilled in the art in viewof the foregoing description. Accordingly, this description is to beconstrued as illustrative only and is for the purpose of teaching thoseskilled in the art the best mode of carrying out the invention. Detailsof the embodiments may be varied without departing from the spirit ofthe invention, and the exclusive use of all modifications which comewithin the scope of the appended claims is reserved.

What is claimed is:
 1. An apparatus for controlling voltage and currentin a switched LED array that is energized via a switched voltageconverter, comprising a semiconductor switching device for connectingthe LED array to a voltage biasing rail from the switched voltageconverter; a current detecting means for generating a signalproportional the current being conducted through the LED array during anactivation time; and a sample and hold means for retaining the detectedsignal during an inactivation time.
 2. The apparatus according to claim1, further comprising a synchronizing means for aligning the activationtime of the LED to the activation time of the switching converter. 3.The apparatus according to claim 1, wherein the switched voltageconverter is a flyback converter.
 4. The apparatus according to claim 1,wherein the switched voltage converter is a push-pull converter.
 5. Theapparatus according to claim 1, wherein the switched voltage converteroperates on an input voltage that is DC voltage signal.
 6. The apparatusaccording to claim 1, wherein the switched voltage converter operates onan input voltage that is a rectified AC voltage signal.
 7. The apparatusaccording to claim 1, wherein the current detecting means comprises aresistor.
 8. The apparatus according to claim 7, wherein the currentdetecting means further comprises an amplification means for boostingthe signal level of the detected signal.
 9. The apparatus according toclaim 1, wherein the sample and hold means comprises a diode, acapacitor, and a resistor.
 10. The apparatus according to claim 9,wherein the response time of the sample and hold means is longer than afrequency of operation of the switched voltage converter.
 11. Theapparatus according to claim 1, wherein the synchronizing meanscomprises a means for inhibiting the operation of the switched voltageconverter.
 12. An apparatus for transferring energy from a source to aload, comprising: a first semiconductor switching device; an energystorage element coupled to the first semiconductor switching device; asecond semiconductor switching device coupled to the energy storageelement; a timing control means for activating the first and secondsemiconductor switching devices; and a synchronizing means for aligningthe time of activation and deactivation of the first and secondsemiconductor switching devices to a predetermined time.
 13. Theapparatus according to claim 12, wherein the energy storage element is acapacitor.
 14. The apparatus according to claim 12, whereinsynchronizing means comprises logic circuits activated by a single logicsignal.
 15. A method for synchronizing a switching LED driver to aswitched voltage converter having an activation time period and aninactivation time period, comprising the steps of: a) activating theswitching LED driver simultaneously with an activation time period ofthe switched voltage converter; b) storing a signal corresponding to thepeak current conducted through the LED during that activation timeperiod; c) inactivating the LED driver simultaneously with theinactivation time period of the switched voltage converter; and d)inhibiting the discharge of the stored peak current signal.
 16. Themethod according to claim 15, wherein the step of storing of the peakcurrent signal is implemented as a sample and hold circuit.